a) Field of the Invention
Laser distance-measuring instruments are based on the principle of pulse transmit time measurement or of phase transmit time measurement.
The phase transmit time method is currently used exclusively with visible semiconductor lasers. It is possible here to implement the overall functioning of the measuring laser and the sighting laser with a cost-effective semiconductor laser.
The disadvantage of the phase transit time method are the extremely small received signals, and the temporally parallel operation of the transmitting system and the receiving system necessitates a powerful receiving system with extremely low crosstalk, transmitting system to receiving system.
b) Description of the Related Art
The distance-measuring instruments known from the prior art which are based on light transmit time measurement can be distinguished in their basic arrangement of transmitting channel and receiving channel into devices in which the transmitting channel is arranged next to the receiving channel, that is to say the optical axes run parallel to one another at a specific spacing, and into devices in which the transmitting channel and receiving channel are arranged coaxially with one another, that is to say their optical axes coincide.
Optical crosstalk, for example owing to backscatter from dust particles in the near zone, can be reduced optically only by two measures: the reduction of the receiver surface, and the axial spacing-of the transmitter and receiver is enlarged. However, the effect of this in the distance range is a rapid migration of the received beam from the receiver. For distances in the near zone, use is made of arrangements with coaxial transmitting and receiving channels, that is to say the transmitting lens, which can also be a single lens, also constitutes the receiving lens. Located within the focal length of this lens is a beam splitter which has the result that the focal plane of the lens is produced in two mutually conjugate planes. Located in these focal planes are the transmitter, on the one hand, and the receiver, on the other hand, with the result that the measuring beam emanating from the transmitter, collimated by the lens, is reflected by the object and is always imaged on the receiver independently of the distance of the object.
This arrangement is suitable for the near zone, since because of the relatively high intensity of the measuring radiation reflected onto the receiver by the object
the pick-up angle of the lens, optimized for emitting the measuring beam, suffices for receiving the reflected measuring beam, PA1 the dynamic range of the receiver is set such that a reflection of the measuring beam on dust particles is not detected, and PA1 a loss in intensity owing to the beam splitter is not a problem. PA1 the pick-up angle of the receiving lens must be selected larger than the pick-up angle of the transmitting lens, PA1 the dynamic range of the receiver is set such that a reflection of the measuring beam on dust particles would be detected when these beam components impinge on the receiver. This is avoided by the spacing of the optical axes of the transmitting and receiving channels and by a small receiver surface, and PA1 no additional loss in intensity is produced by a beam splitter. PA1 the transmitter power, PA1 the loss in intensity over the length of the beam path, equal to double the distance of the object, and PA1 the respectively effective aperture range, that is to say the fraction of the surface of the receiving lens which is effective for imaging the reflected measuring beam on the receiver, in each case.
Because of the low intensity of the reflected measuring beam and the relatively high intensity from the near zone, caused by the optical components (beam splitter, lens), and the dust particles, this arrangement is unsuitable for the far zone.
The parallel arrangement of transmitting and receiving channels is selected for the far zone, that is to say the object to be measured is located at infinity for the receiving lens, which can also be a single lens. Since the measuring spot produced on the object to be measured is always imaged coming from infinity at the focus of the receiving lens, it is possible to dispense with arranging the transmitter and receiver in mutually conjugate planes, and this permits the transmitting and receiving channels to be separated.
This arrangement is suitable for the far zone, since because of the relatively low intensity of the measuring beam reflected onto the receiver by the object
This arrangement is unsuitable for the near zone because of the parallax produced, which has the effect that as the distance becomes shorter the image of the measuring spot increasingly migrates away from the receiver arranged on the optical axis of the receiving lens.
Taken together, the above statements make it difficult to imagine designing a laser distance-measuring instrument which is suitable for a large distance range. A large distance range is to be understood as a range which comprises both the near zone and the far zone.
The need for such distance-measuring instruments exists, for example, in the construction sector, where a distance range of 0.3 to 30 m is of interest.
Because of the reduction in intensity in the case of the coaxial arrangement, an arrangement with parallel transmitting and receiving channels comes into consideration for a large distance range. Such an arrangement is disclosed in EP 0 701 702 B1. In the laser distance-measuring instrument described here, two basically different solutions are offered, so that even in the near zone the measuring spot is always imaged on the receiver, here the optical conductor entrance surface.
This can be performed, on the one hand, by tracking the optical conductor entrance surface in accordance with the displacement of the imaging position of the measuring spot, specifically only transverse to the optical axis. As specified in the patent, there is deliberately no tracking along the optical axis, since it has emerged that tracking into the concrete image position leads to overdriving of the evaluation electronics, that is to say the dynamic range of the receiver, for which the control electronics are designed, is exceeded.
On the other hand, it is proposed to arrange the optical conductor entrance surface in a fixed version and to ensure by means of optical deflecting means arranged outside the optical axis that the measuring beams entering the receiving lens ever more obliquely in the case of short object distances are directed to the optical conductor entrance surface. Here, as well, it is assumed that it is not a deflection which is correct in terms of imaging optics which is important, since there are no intensity problems in the case of close object distances. The second-named variant has the advantage that it manages without mechanically moving elements in the receiving channel.
However, it has the disadvantage that it is scarcely possible for the signal level (intensity of the measuring beam impinging on the receiver and reflected by the object) to be matched to the dynamic range of the receiver.
If suitable measures ensure that a portion of the measuring beam reflected at the object impinges on the receiver surface, the distance-measuring range is limited by the sensitivity range (dynamics) of the receiver.
The following are essentially decisive for the intensity of the radiation impinging on the receiver surface: